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AJ Handbook of Building Structure

EDITED B Y

Allan Hodgkinson

The Architectural Press, London



AJ Handbook of Building Structure Introduction

Allan Hodgkinson

Consultant editor and authors

The consultant editor for the Handbook is Allan Hodgkinson

MEng, FICE, FIStructE, MConSE, Principal of Allan Hodgkin-

son & Associates, consulting civil and structural engineers.

Allan Hodgkinson has been the AJ consultant for structural

design since 1951; he is a frequent AJ contributor and is the

author of various sections of this handbook.

The authors of each section will be credited at the start of

the section of the Handbook in which their material appears.

The original Architects' Journal articles were edited by

Esmond Reid, BArch, and John McKean, BArch, MA, ARIBA,

ACIA, ARIAS.

The frontispiece illustration shows one, of the most

magnificent building structures from the era of the Eiffel

Tower, the Forth Bridge and the great railway stations.

The Palais des Machines for the Paris Exhibition of 1889

(Contamin, Pierron & Charton, engineers) was a pioneer

example of three hinged arches.

Preface to the second editionThere have been considerable changes in some British

Standards, Codes of Practice, and Building Regulations

since 1974; and unlike the reprints of 1976 and 1977, this is

a substantially revised and updated re-issue of the now

well-established AJ Handbook of Building Structure.

The principal changes are in the sections on Masonry (re-

written to take account of the 1976 Building Regulations,

and the new BS 5628 'limit state' code of practice); and on

Timber (substantially revised to take account of the new

timber gradings).

Steel handbooks have been replaced for all types of struc-

tural sections; and technical study Steel 3 has therefore

been revised accordingly.

In general, the new 'limit state' approach to design is dis-

cussed (eg in the section on Masonry); but in view of the

rejection of the limit state Codes and draft Codes in their

present form, by the majority of practical designers, it has

been thought prudent to retain the allowable stress methods

of design as the basis of the handbook.

Finally, it should be mentioned that the opportunity has

been taken to bring all references in this Handbook up to

date; and to correct a number of misprints of the first edition.

ISBN 0 85139 273 3 (paperbound)

First published in book form in 1974 by

The Architectural Press Limited: London

Reprinted 1976, 1977

Second edition 1980, 1982, 1983

Printed in Great Britain by

Mackays of Chatham Ltd

This handbook

Scope

There are two underlying themes in this new handbook on

building structure. First, the architect and engineer have

complementary roles which cannot bo separated. A main

object of this handbook is to allow the architect to talk

intelligently to his engineer, to appreciate his skills and to

understand the reasons for his decisions. Second, thebuilding must always be seen as a whole, where the success-

ful conclusion is the result of optimised decisions. A balance

of planning, s tructure or services, decisions may not neces-

sarily provide the cheapest or best solution from any of

these separate standpoints, but the whole building should

provide the right solution within both the client's brief and

his budget.

The handbook provides a review of the whole structural

field. It includes sections on movement in buildings, fire

protection, and structural legislation, where philosophy of

design is discusssed from the firm base of practical experi-

ence. Foundations and specific structural materials are also

covered, while sufficient guidance on analysis and design

is given for the architect to deal with simple structures

himself.

Arrangement

The handbook deals with its subject in two broad parts.

The first deals with building structure generally, the second

with the main structural materials individually.

The history of the structural designer and a general

survey of his field today is followed by a section on basic

structural analysis. The general part of the handbook

concludes with sections on structural safety—including

deformation, fire and legislation—and on the sub-structure:

foundations and retaining structures.

Having discussed the overall structure, the sections in thesecond part of the handbook discuss concrete, steelwork,

timber and masonry in much greater detail. Finally there

are sections on composite structures and on new and

innovatory forms of structure.

Presentation

Information is presented in three kinds of format: technical

studies, information sheets and a design guide. The technical

studies are intended to give background understanding.

They summarise general principles and include information

that is too general for direct application. Information sheets

are intended to give specific data that can be applied

directly by the designer.

Keywords are used for identifying and numbering technicalstudies and information sheets: thus, technical study

STRUCTURE 1, information sheet FOUNDATIONS 3, and so on.

The design guide is intended to remind designers of the

proper sequence in which decisions required in the design

process should be taken. It contains concise advice and

references to detailed information at each stage. This might

seem the normal starting point, but the guide is published at

the end of the handbook as it can be employed only when

the designer fully understands what has been discussed

earlier.

The general pattern of use, then, is first to read the relevant

technical studies, to understand the design aims, the

problems involved and the range of available solutions.The information sheets then may be used as a design aid, a

source of data and design information. The design guide,

acting also as a check list, ensures that decisions are taken

in the Tight sequence and that nothing is loft out.



Section 5

Structural material: Reinforced concrete

ScopeSections 1 to 4 of the handbook reviewed design generallyand gave a résumé of constructional materials followed bysome basic, analytical theory. Safety, statutory requirementsand foundations were examined. The succeeding sectionsdeal with each major structural material in turn so thatthe architect, having digested the properties of each materialand the variety of structural form in use, can relate theinformation logically to his own design problem.

The perfect structure does not exist in building. It wouldneed a form with minimum internal forces, most efficientstructural shape and using the minimum, cheapest materialgiving at once durability, insulation, resistance to fire andoccasionally to noise,. and not requiring maintenance.Obviously good design achieves a compromise of all theseaspects to satisfy the clients’ brief. While some aspects of the design process may be a reasonably exact science, the

whole is very much a question of general intelligence andintuition, sometimes referred to as ‘thinking with the hips’. The handbook concludes with a comprehensive designguide which attempts to both pose the problems andsummarise the solutions.

Author The consultant editor of the handbook and author of thissection is Allan Hodgkinson MEng, FICE, FIStructe, MCOnSE,a consulting engineer.

Allan Hodgkinson

Illustration over page shows board -marked concrete

lift shafts



1 Materials

1.01 Reinforced concrete is now so commonly used that ittends to be compared with other structural materials as if

it were, like them, homogeneous. In fact, not only is thereinforced concrete member a composite construction withsteel but the concrete itself is a composition of cement, fineaggregate and coarse aggregate. TableI shows the range of concretes used structurally in this country. Most of theconcrete used in bidding is composed of ordinary Portlandcement, and sand with gravel or crushed stone, producing adensity of from 2300 to 2400 kg/m3 and cube strength of from 21 to 27 MN/m2. Aerated concrete is an additionalstructural material but is only available in the precast formowing to the method of manufacture.

Cement1.02 Cement is a material with adhesive and cohesiveproperties which enable it to bond mineral fragments. Thecements used in bidding have the property of setting andhardening under water and are usually described as hydraul-ic cements, with the sub-classifications natural, Portlandand aluminous. They consist mainly of silicates and alu-

minates. Portland cement, the most commonly used, ismanufactured from limestone or chalk; and the alumina andsilica from clay or shale. Manufacture consists of grindingand mixing the raw materials and burning them in arotary kiln at about 1400°C until the material fuses into

balls of ‘clinker’. The clinker is cooled and ground to apowder, some gypsum added, and the final product storedin silos at the works. Distribution is usually in 50 kg bagsor in bulk to silos on site. If delivered too early ‘hot’ cement

may roach the site with adverse effects on the concrete.1.03Ordinary Portland cement,specification toBS 12, com-

prises 90 per cent of the cement used in Britain. BritishStandards relating to chemical composition, control the limesaturation factor and the magnesium and gypsum content.Strengths are consistently higher than prescribed.1.04 Rapid hardening Portland cement, specification toBS 12, is similar to ordinary Portland. The acceleration isachieved by finer grinding and an increase in the tricalciumsilicate content. Assuming the same water/cement ratio,rapid hardening produces 7 day strength equal to thatproduced by ordinary cement at 28 days. Extra rapidhardening Portland cement is produccd by intergrindingrapid and calcium chloride. Strength is about 25 per centhigher than rapid at 1 to 2 days, and 10 to 20 per cent,higher at 7 days. Setting time depends on temperature andcan be as little as 5 to 30 minimum after mixing. This makesearly placing vital.1.05 Low heat Portland cement, to specificationB Y 1370,produces an ultimate strength equivalent to ordinaryPortland but is less finely ground arid is low on the morerapidly hydrating compounds, resulting in slower develop-ment of strength arid lower rate of heat development. It isused primarily for bulk placing of concrete in dams or

159 Technical study Concrete 1 para 1.01 to 1.05

Technical study

Concrete 1Section 5 Structural material: reinforced concrete

Concrete: the material

and its properties

Having covered the theory and practice of structural design,

the handbook now turns to a more detailed inspection of the

various structural materials, their history, their properties

and their present effects on design. This is the

first of three studies on concrete by ALLAN HODGKINSON.

Here the various types of concrete are described with their

basic constituents and relative properties. The next study will

look at the uses of concrete as a structural form

Coarse aggregate

Crushed stone, mainly

limestone and granite,

(or) gravel, blast

furnace slag

Foamed slagExpanded clayExpanded shale

Expanded slateSintered PFA

Barytes

Steel }Lead

Fine aggregate

Sand or

pulverised fly ash

Sand or

crushed stone

CrushedCrushed

Crushed

Crushed

Crushed

If sand is used

extra strengthmay be obtainedwith extra density

Coarse

aggregate

bulk densitykg/m3

1380-1620

550-800

2500-3000

depends on

shape, usuallyspheres

Concrete

density

kg/m3

540-800

2000-2400

1850-19801500-1600

1450-1650

1600-1800

1550-1710

3400-3600

Compressive

strength

MN/m2

3.5-5.5

14-100

12-48

13-33

15-42

13-50

18-56

21

70

Conductivity

'K '

W/m 2

3-5

30-32

10-13

Particular use

Aerated concrete in building blocks

and reinforced precast slabs

General structural use in the range 20-30.

Mass and reinforced and prestressed

concrete in situ or precast concrete.

High strength building blocks

General struct ural use in the range 20-30

Reinforced and prestressed concrete.

Building blocks

Special applications such as

radiation shielding

Table I Range of concretes used in building structures in the UK



Technical study Concrete 1 para 1.05 to 2.01

large foundations where the heat of hydration mightdamage the setting concrete.1.06Portland blast furnace cement, to specificationBS 146,is made by intorgrinding Portland cement clinker andgranulated blast furnace slag. The slag, a waste productfrom pig iron manufacture, is a mixture of lime, silica andalumina, the same oxides which make up Portland cementbut in different proportions. The resulting cement is

similar to ordinary Portland but has a lower heat of hydra-

tion and hardening rate.1.07 Sulphate resisting Portland cement, to specificationBS 4027, has a lower tricalcium silicate and tetracalciumaluminoferrite content than ordinary Portland, thusincreasing the resistance to sulphate attack. Early strengthis low and ultimate strength high. It develops only a littlemore heat than low heat Portland.1.08Supersulphated cement, to specificationBS 4248, is nota Portland cement. It is made by intergrinding a mixtureof granulated slag, calcium sulphate and Portland cementclinker. It is highly resistant to sea water, to the highestconcentrations of sulphates normally found in soil orground water, to oils and to the acids normally found with

peat. At normal temperatures it has low heat of hydrationand slow gain of strength. It must not be steam cured ormixed with other cements or additives.1.09High alumina cement, to specificationBS 915, consistsof about 40 per cent each of alumina, and lime, with someferrous and ferric oxides and about 5 per cent of silica.Raw materials are limestone or chalk and bauxite. UnlikePortland cement the materials are completely fused in thokiln, emerging a5 a molten material which is solidified,fragmented in a rotary cooler and ground. The result is adark grey powder more in the nature of a chemical than aPortland cement. Although the cement is slow setting, itdevelops about 80 per cent of its ultimate strength within24 hours. Resistance to chemical attack is high and, as with

supersulphated, it must not be mixed with other cements oradditives. The greatest care is required in its application andits use has been banned in structures.1.10Coloured Portland cementsare available to specificationBS 12. White cement is made from raw materials containingvery little iron and manganese oxide. China clay is usuallyemployed together with limestone or chalk free from im-

purities. Other colours are obtained by intergrinding whitecement with various pigments. About 10 per cent morecoloured cement is usually required to achieve the concretestrength provided by ordinary Portland.1.11 Hydrophobic cement is obtained by intergrindingPortland cement with either oleic acid, stearic acid orpentachlorophenol. The hydrophobic properties are due to

tho formation of a water repellent film around each cementparticle. The film, which remains intact until mixing,protects the concrete even during long storage in unfavour-able conditions.

Table II indicates the relative costs of a nominal 1:2:4 mixconcrete between a variety of cements and an average sandand gravel aggregate.

Table II Approximate cost of 1 m3 1:2:4 gravel concretewith various cements, placed in an rc beam. Assumes

batches of over 10 tonnes and delivered within the London area(March 1979 prices)

160

Aggregates1.12 The most commonly used aggregates are described intable I. The lightweight variety produce concrete with adensity of 1500-1800 kg/m3 while the normal weightproduce 2000-2400 kg/m3. Lightweight aggregates aremanufactured in a limited number of areas and transporta-

tion costs must be borne in mind before opting to use them.1.13 Natural gravels and crushed rocks are' still plentiful

throughout the country and in certain areas sea-dredgedaggregates are available. Aggregates from natural sources

are normally specified to comply with BS 882 and to besampled and tested in accordance withBS 812.1.14Consideration should be given to chemical and mineralcomposition, specific gravity, hardness, strength andphysical and chemical stability. All of these may be regardedas properties of the parent rock. Properties particular to theaggregate are the particle shape and size, surface textureand absorption. All these properties may influence theconcrete quality in both the fresh or hardened state. A goodaggregate will usually produce good concreto.1.15 Grading is achieved either by mixing screened singlesizes or by stockpiling the various sizes on site and mixing

by weight according to the requirements of a designed mix.Sea-dredged aggregates should be specially checked forshells and chloride salt content. A quite high shell contentis permissible, but the inclusion of large shells containingsoft sand can be a problem, particularly in fairfacedwork.1.16 Lightweight aggregates must comply with BS 877,3797 and 3681. These include foamed blast furnace slag,expanded clay and shale, expanded slate and sinteredpulverised fly ash. The fine aggregate obtained by crushingtends to be angular while only two types of coarse aggregateare rounded. The strength of the aggregate places a maxi-mum value on tho strength of the concrete in which it isemployed.

Water1.17 This essential ingredient should comply with BS 3148and be clean and free from impurities. If it can be drunk, itis acceptable.

Admixtures1.18Admixtures are not yet subject to a British Standard.Calcium Chloride at restricted percentages was the mainbasis of acceleration mainly for use in cold weather but itsuse with reinforcement is banned. Proprietary materials aremarketed to improve the nature of both wet and hardenedconcrete, Additives improving the hardened concrete maygive an increase in control of heat hydration and expansion

action, increased resistance to water penetration andchemical or fungicide attack and greater corrosion inhibition.Air entrainment in the wet concrete can give weight reduc-tion and greater frost resistance in the hardened concrete. If high early strength is not essential, pulverised fly ash maybe added in place of some of the cement. Admixturesshould not be used indiscriminately. A well designedconcrete mix with good site control and placing may wellprovide the answer.

2 Mix design

2.01 In the case of ready mixed concrete plants and precastconcrete works, a range of mixes will be established on the

basis of supplies from particular sources. For the on-sitesupply of concrete, there is the possibility of the use of nominal volume mixes, standardCP 114 mixes by weight, ordesigned mixes. A nominal mix could vary in strength fromplace to place and might not necessarily fulfil the design

Cement

Ordinary Portland

Rapid HardeningExtra rapid

Sulphate resisting

Super sulphatedAluminousWhite

Concrete cost £/m3

37.55

38.0038.75

40.30

43.5046.80

53.70



2.03 The materials are variable and properties cannot beassessed precisely so that the end product is not so much adesign as an intelligent guess at something near the answer.Hopefully the number of trial mixes leading to the rightsolution can be kept to a minimum. Such trials are usuallyat laboratory level so it is still necessary to verify on sitethe results from the bulk materials and the particularmixing plant and placing methods.2.04Some basic facts guide the designer:1 A specific amount of water is needed to hydrate thecement. Extra water is needed only to provide workability.2 The strength of the cement paste relates directly to theamount of water used. Excess water reduces the strengthand detracts from a number of other properties.3 The cement paste covers the surface of tho aggregate andfills the voids between the particles. The largest aggregatesize with appropriate subsidiary gradings will therefore usethe minimum paste. In foundation work it is common touse aggregate of maximum size 40 mm, but, in some re-

inforced concrete members it may reduce, to as little as9 mm in order to get the concrete around the reinforcement.4 Workability may be improved by plasticisers.2.05 The Code of Practice CP 110 tabulars a number of mixes by 28 day cube compression strength. Of these grades20 and 25 suffice for the greater part of structurall concreteused in buildings. Table IV shows the cost for varyingstrengths of concrete using ordinary Portland cement withaverage sand anti gravel aggregate. These basic costs wouldlikely be higher using a lightweight aggregate but theother properties and the consequential effects on the whole.

Table IV Cost of concretes with varying strength placed in bulk

(March 1979 prices)

requirements of tightness, minimum shrinkage or abrasion.2.02 In building work, strength is the usual criterion andis usually in accordance with Road Note 4 (HMSO, 1970). Table III shows the headings under which mix proportionsshould be considered.

assessed in order to make a fair comparison.2.06For a designed mix for normal structural concrete, thespecification should define strength grade, type of cement,minimum cement content in kg/m3 of finished concrete,nominal maximum size of aggregate, maximum water/cement ratio and the required workability. It is impossibleto define the precise proportioning and then expect to hold

the contractor responsible for the end result. Should specialrequirements be necessary, as mentioned earlier, the entiremix should be specified on a performance basis or specialtests introduced.

3 Testing and acceptance

3.01A variety of tests specified inBS 812 andBS 3681 checkthe aggregates as received on site; BS 1881 defines thetesting procedures for both fresh and hardened concrete. These checks relate primarily to aggregates and concretoworkability as cement in this country is sufficientlyreliable to be covered by works certificates. Tho only likelysources of trouble here are overlong storage or too early

delivery from the works.3.02Aggregate tests include: sieving to check the grading; asilt test to determine the extent of clay, silt or fine dust;an organic impurities test, primarily for sands; tests forbulking of the sand and bulk density of the aggregates;and a check on moisture content. Also to be checked are thestorage condition for materials, the cement store or silo,the aggregate stock-piles, the water supply and the clean-liness and efficiency of the concrete batching plant andmixer.3.03 The two basic checks on mixed concrete are on theworkability using slump cone or compacting factor appara-

tus, and the making of 150 nun or 100 mm cubes. Thesamples for making cubes should always be taken at tho

point of placing. Modern specification should require.coinpaction by vibration, in which case a slump of say 25 to50 mni is appropriate to mass or lightly reinforced sections,and 50 mm to 100 mm for more heavily reinforced sections.

The specification map demand a minimum 28 day orperhaps 7 day strength. Required values and test methodsare given in CP 110 andCP 114.3.04 Failure to comply does not necessarily mean thatconcrete placed previously has to be cut out of the structure,provided that consistent results have been obtained beforeand that tests have shown tho cement and aggregates to besatisfactory. The cubes and their method of making andstorage should be examined in case the test sample itself was at fault. There may also be ample reserve in tho design

to accommodate a slight understrength in a structuralmember. Non-destructive testing may be required on a partof the finished structure. This is dealt with in BS 4408 underthe headings of 'gamma radiography', 'surface hardnessmethods' and 'ultrasonic pulse velocity'.


Strength at 28 days MN/m2

2025 }30

40

50}

60

Concrete cost £/m3

37.30

38.60

40.30

45.35

4 Distribution and placing

4.01 Distribution on site is the contractors' problem. It may

be by pumping, by blowing, by skip and crane, or simply

by dumper to concrete hoist, barrow and chute. Workability

will vary to allow for the method employed. Satisfactory

concrete once produced has still to be properly placed and

compacted. A small reduction in compaction will mean a

considerable loss of strength. Distribution and placingmethods must be checked and further care taken as the

concrete hardens and matures. This is particularly important

in extremes of hot or cold weather. In the former there is a

curing problem and in the latter the concrete could, at

desiqn

strengthhaving regard

to quality

control

type of cement

having regard topossible att ack &

rate of gain of

strength

durability

type of

compaction

aggregate

type

water/cement ratio workability

aggregate/cement

ratiooverall aggregate

grading

aggregate

size &

shape

proportions of

single sized

aggregatesmix

proportions



Technical study Concrete 1 para 8.01 to 9.04163 Technical study Concrete 1 para 8.01 to 9.04

8 Steel reinforcement

8.01 The manufacture of steel is dealt with in section (i of

the handbook. Steel reinforcement is only one type of

steel fabricated by the mills. Shaped bars may be taken

from the mills and their properties transformed by the

pulling and twisting effects of cold working. British Stand-

ards cover different types of reinforcement such as1 Hot rolled steel bars (BS 4449)

2 Cold worked steel bars (BS 4461)

3 Steel mesh fab ric (BS 1221)

4 Plain hard-d rawn steel wire for prestressed concrete

(BS 2691)

.5 Indented or crimped stool wire for prestressed concrete

(BS 2691)

8.02 Steel for ordinary reinforced concrete has an ul timate

strength range from 250 MN/m2 to 410 MN/m2, though

proprietary steels with special bond control features go

from 550 to 680 MN/m2. Elongation for 250 steel is 22 per

cent and for 410 steel is 14 per cent. Ultimate tensile

strength must be at least 15 per cent greater than yield.

Preferred bar sizes in mm diameters are:6 8 10 12 16 20 25 32 40. 16 and 20 mm give the basic

price with bars of greater or smaller diameter becoming

increasingly expensive. Lengths greater than 12 m are also

more expensive. Generally the cold worked steel is economic

for structural application and tends to be used even in

positions where mild steel would suffice, if only to avoid

confusion on site. Hot rolled steel to BS 449 is used as

an alte rnat ive. British Standards require 460 grade

bars up to 16 mm to have a minimum 12 per cent elonga-

tion and the 425 grade bars over 16 mm to have a mini-

mum 14 per cent elongation.

8.03 Steel wire for prestressed concrete is supplied in plain

or indented form with either normal or low relaxation—a

reference to the creep characteristic in an ultimate

strength range of 1470-1720 MN/m2 with mini mum 0 .2 per

cent proof stress (in this case the equivalent of yield point)

of 1250-1550 MN/m2. Preferred sizes are 4, 5 and 7 mm

diameter. Proprietary alloy steel round bars may be used

as an alternative to stool wire in cable or strand form.

8.04 The majority of building structures are designed in

normal reinforced concrete and the reinforcement is largely

used in tension. As the hot rolled or cold worked high

tensile bar costs only about 8 per cent more per tonne than

mild steel, and has only two-thirds the equivalent weight,

it is far more economic, so much so that it still tends to be

used in the few cases where it is not the cheapest answer.

9 Advances in concrete

9.01 Radical innovation is unlikely to occur in a material

dating from the Roman era, but refinement is possible. The

last 30 years have seen improvement in materials and

manufacture without any change of the basic ingredients,

leading to stronger, cheaper, better concrete. The precast

factory can produce extremely high strength concrete on

account of the closer controls possible and the greater

scope for applying techniques in placing, compaction and

curing.

9.02 Strengths of 100 MN/m2 are feasible using carefully

selected materials, but only at higher cost. This would be

of doubtful advantage in in situ work, even for localisedareas of high stress, on account of the need for varying

grades of concrete on site. It is in the long span structures,

for example bridges, that such practice will bring overall

economy.

Resins

9.03 Resins are used in three ways. They may act as a base

for the concrete instead of cement and water. Resins lose

strength at high temperatures so this type of concrete

cannot be used in a fully structural capacity, only in semi-

structural applications such as machine bedding and bolt

setting. Resin impregna tion is the fill ing of the voids in a

normal mix with a polymer and then curing the polymer.This is expensive and has practical difficulties, but is being

actively researched in the us. Resin additive concrete is

produced by adding a liquid opoxide or polyester which

cures by drying out or by chemical reaction. About 10 per

cent of the weight of the cement is required before the

resins can give worthwhile improvements. This virtually

restricts their use to thin screeds.

Fibre reinforcements

9.04 The success of asbestos cement and glass fibre products

has prompted research int o fibre-reinforced concrete. This

is in an effort to achieve a near homogeneous member

instead of the compressive-tensile composite that is rein-

forced concrete, but it is unlikely to be economic. It wouldrequire about ten times the reinforcement weight of the

conventional method in order to give the equivalent

flexural and tensile strength. Small amounts may be used

to increase impact strength and crack control and to

affect the actual mode of failure in a member. Fibres used

include metal, glass, plastic and carbon. The first two

provide adequate tensile strength with the proviso that

glass, with its low alkali resistance, must only be used with

aluminous, rather than Portland cement. Plastic, such as

polypropylene and nylon, has a low elastic modulus,

confining its use to semi-structural members, for example

cladding. Carbon fibres are currently too expensive to be

considered. Generally fibres are difficult to handle. Both

mixing and vibration pose problems so this again is a

process for factory, not site.

For the time being it would appear that the well-tried

methods and materials will continue in use, particularly

for work on site.

Flint-faced Roman concrete, Burgh Castle, Suffolk

Structural concrete spans, Seaton Bridge, Devon (built 1876).



Technical study Concrete 2 para 1.01 to 2.03 164

Technical studyConcrete 2

Section 5Structural material : reinforced concrete

In his first study on concrete ALLAN HODGKINSON described

the material, its constituents and properties. This study

describes its structural applications

Structural applicationsof reinforced concrete

1 Introduction

1.01 Because of its versatility, design flexibility, andinherent fire resistance, reinforced concrete, particularlyin situ, can be used for a wide range of structures. Members

can vary in shape and size to suit special design and layoutrequirements, and floors can be punctured to accommodateservices, but once cast the structure is unalterable. Thedevelopment of precast concrete has simplified the designand construction of skeletal structures. Prestressing hasextended the spans to which concrete structures can becarried and structural steelwork has been partly deposedeven in bridge building.1.02 In the last two decades experience has proved thatconcrete can assume almost every structural form. However,the relationships between costs of materials, labour andplant vary with time and cause like variations in the cost-competitiveness of one material over another, and within amaterial of one structural form over another.1.03 The designer must constantly bear in mind that theultimate economy to be achieved is that of the building,not of the structure done; so an in situ floor slab which hasa ready-for-decoration soffit and a power-floated top surfaceready to receive a carpet finish may show an overall savingcompared with an apparently cheaper precast assembly

quence is to cast the columns and strip the formwork, erectthe beam formwork and cast the beams up to the undersideof the proposed slab, then strip the beam sides, erect slabformwork and finish the slab. Sometimes it may be conven-

ient to erect shuttering for beams and slabs and place both

in one operation. Then the formwork can be arranged so thatthe beam sides can be removed before the slab formwork(re-use of formwork and speed of re-use are essential toeconomy). Ways of leaving support until the concrete iscapable of carrying its dead and construction load must befound; removing all the formwork and then replacing theprops is a last resort but sometimes the only way. Timeuntil stripping relates to span and loading as well as tostrength maturance of the concrete. There will be occasionswhen construction loads are higher than live loads and somay require support for the ensuing wet concrete throughtwo or more finished levels-apoint which should be madeto the contractor at the time of pricing. Part precastsolutions can be employed in this type of structure in anattempt to cut out the more difficult aspects of the form-

work process. However, the more bays there are in eachdirection, the more difficult does the cranage of heavyprecast members become-placingdimensional restrictionson the use of this system.

2.01 Over the past 20 years reinforced concrete has beenused for most building frames, primarily because the light-weight casing of structural steelwork was slow to developand the solid concrete casing required for fire protectionprecluded structural steelwork as a competitor. Buildingfirms geared themselves to in situ concrete construction of entire frames and floors and though the margin between thetwo materials has lessened, the all-concrete structure is still

the most common. Assuming a building of repeating bays10 m x 7 m, the following solutions have been employed.

Column, main beam, secondary beam and slab2.02 This was acceptable in the 1930s, but is only usednowadays for very heavy loading 1. The construction se-

1 Building frame: column, main beam, secondary beamand slab

Column, beam, slab-one-way span2.03 This is cheaper and now more commonly used 2; butdispensing with the secondary beam leaves the structuredependent on some other way of resisting wind forces atright angles to the main beam line-usuallysome stiff lift

or stair arrangement. The slab has to be thicker to span theextra distance, so to limit its weight without loss of strength,many types of void formers are used and formwork is thenrequired to support the void-forming devices. Thoserequiring a full slab area formwork include hollow blocks,lightweight solid blocks, expanded metal boxes, cardboard

which requires ceiling finish and screed to attain the same

standard.1.04 Superstructure and foundations must be considered

together. A lightweight construction of longer span may

provide a better solution than the apparently cheaper

normal weight structure of shorter span. Thus there is no

simple formula for deciding on a structural type and experi-

ence and intuition play a decisive part in the final choice.

The purpose of this study is to illustrate, and comment on,

the many structural forms which can be used.

2 The typical building frame



prietary telescopic formers which are adjustable to span

between beam edges or brick walls, or on a complete soffitshutter 5.

2.06 The larger void-forming devices such as plywood steel,

fibreglass or polypropylene troughs can produce a very

pleasant soffit appearance which the architect may be able

to use to advantage. A flat soffit may be obtained by pinning

plasterboard to the ribs. The trough can be standard, or

fixed, or if the size of contract makes it worthwhile, pur-pose-made. One solution is shown where minimum form-

work is employed and the trough can be removed after one

or two days 6.

2.07 Formwork can be kept to a minimum and part removed

after one to two days allowing the troughs to drop but

leaving props as long as is necessary to develop the strength

of the rib. Cross ribs result from leaving gaps between the

trough ends, their centring dependent on trough length.

side supports

removed early

main support

removed with propsprop

soffitboard

trough

boxes, expanded polystyrene blocks, and either cylindrical,

cardboard or inflatable-duct tubes 3, 4.

2.04 The hollow pot 3 has done yeoman service for manyyears but its high labour content and breakage rate nowtend to make the equivalent solid slab cheaper. Load to becarried to foundation must always be kept in mind but thelabour content of the solid slab is less and it gives more flexi-bility where lift or lightwells or variable heavy loading occur.A cautionary word about the use of horizontal slip tilesbetweon pots as permanent shuttering for the undersidesof the beams—there is a danger of poor compaction betweenthe underside of the reinforcement and the top surface of the tile and this can remain hidden. If slip tiles are used adeep cover should be allowed. Preferably expose the ribsoffit by not having a slip tile and make clear to the con-tractor the problem of spacing the blocks when he tenders.Where spacing is a deflection constraint, perhaps owing to

plaster finishes below or to sensitive machinery requiringlevel support, remember that a ribbed slab normally has30/26 deflection of the solid slab. Thus allow an equivalentdepth increase where minimum slabs are being compared.Where heavy point loads occur, cross ribs should be intro-

duced. These act as load spreaders by forcing the main ribs

to deflect together. Void formers buried in the depth of theslab give a performance almost equivalent to the solid slab,but locating the formers is difficult, for in addition to theproblems of placing and vibrating, the former tends tofloat in the wet concrete 4.

6 Trough formwork can be struck leaving only temporarysupport as beams attain full strength

Column, beam, slab—two-way span2.08 With beams on each grid line the slab spans in bothdirections 7 and therefore can be thinner. The beam rein-

forcements are of the same order, and intersect at thecolumn head. The top and bottom steel layers of adjacentslabs also converge at this point, and the resulting con-

gestion places a limit on minimum slab depth. This form of construction has taken on a new lease of life since the fifthamendment of the Building Regulations came into force,because the two-way system provides enough reinforcement

at enhanced allowable stresses to span the structure oneway were a beam to be removed.

7 Building frame: column, beam slab---two-way span

2.09 As the spans increase, void formers can be employed toturn the dab into a two-way ribbed construction. Tho voidscan be formed by steel, glass fibre, or polypropylene wafflepans and by the use of cardboard boxes, expanded metalboxes or expanded polystyrene blocks.2.10 Waffle pans have recently been standardised and can behired, though as with troughs, a large contract may permituse of purpose-made units Again depending on the type

of former the soffit formwork may be an all over or partlyremovable system. With the waffle pan supported on twosides only, it is possible to use the method employed withthe trough, provided the unsupported, open edge of thepan is taped.

5 Pressed concrete blocks as permanent soffit can betemporarily supported by triangular section telescopic

formers spanning between beamedges or brick walls

2.05 Pressed concrete blocks can be supported either by pro-



166

reducing tho effective span of the slab. The flat slab andsimple column provides the cheapest formwork arrange-ment but the available shear perimeter is quite small.2.13 The drop slab produced two critical shear conditions11. At G the total panel shear is carried on a perimeterdetermined by D and d1, and at 7 by Ldrop and d2. In thesame way the capital increases the slab shear perimeterrelative to the maximum diameter of tho capital head, and

where tho capital arid drop are used together the criticalsection within the drop is raised yet again and very heavyslab loads can be carried. As in the two-way span supportedby beams, the floor weight can bo reduced by the use of waffle pans provided that there is sufficient solid concretearound the column head to deal with shear forces.2.14 It is possible to reinforce the area around the columnhead by the use of welded crossheads in structural steel-work or by using a system of bent-up bars-a method to beused with caution and as a last resort, owing to practicalproblems of placing the reinforcing steel.

10 Shear perimeter in slab using square and circular section

columns

9 Flat and drop slabs: column capitals and slab drops(2 to 5) reduce effective span of slab and transmit shearinto column head

11 Drop slab : shear perimeter shown dotted

Flat and drop slabs Flat slab and banded plate

2.11 An empirical method can be applied to flat slab design 2.15 The idea of abandoning the beam and making a widestrip of slab do the work evolved from the mid-1940s. Firstcame the wide shallow beam with slightly simpler formwork,easier concrete placing and less overall height to the

building, then in the early 1950s the beam disappearedaltogether.

illustrated in 12 it is necessary to look at finishes and thebuilding cube as well as at the structure itself. In the struc-ture the amount of reinforcement will vary almost linearly

pared with slab 3, extra formwork costs in the sides andsoffit and extra plaster and decoration costs.

provided the plan form lies within certain limits 8, 9.L1 must not exceed 1.33 x L2. Adjacent panels L1 and L2

dimensions must not differ by more than 10 per cent. Endspans may be shorter but not longer than interior spans.

The thickness of the slab should be constant. There should

angles. Generally, a nearly square grid analysd by theempirical approach product an economical answer forquite heavy loading conditions. This method does assume

column. However, in the case of car parking floors, whichwork quite well on a 8 m square grid, the ideal column sec-tion is oblong to facilitate parking. In its weaker direction itcould not accept the empirical moment, but given analternative analysis and allowing only for the out of balancelive load moment between adjacent spans it can be made to

work.2.12Column capitals and dropped slabs10are used for thedual purpose of getting the shear into the column head and

be at least three rows in each of two directions at right 2.16 To consider the overall costs of the three schemes

that a fairly high arbitrary moment is taken into the with d1, d2 and d3. Items 1 and 2 have extra concrete com-

2.17 If headroom beneath the beams is to be the minimum

headroom in the building, as is usually the case, then the

extra beam/stem depth must be added for every floor —in a

10-storey building this could be 5 m. The extra effect on the

building cube is considerable and may result in the loss of a

floor where overall building height is under planning

control. An added advantage of the flat ceiling in office

buildings is that there can be complete flexibility of parti-

tions. However, the span of the solid slab is limited, especiallywhen it has to double as a beam. The beamless floor deflects

more and brittle partitions such as lightweight blocks and

plaster must be used with care (see Technical study SAFETY

2, AJ 7.6.72).

2.18 By using void-forming devices spans can be improved

a b c

12a Beam and slab; b drop slab; C flat slab




without increase in weight and with the greater depth little

increase in reinforcement. Making use of the trough or

waffle pan can strike a balance between floor types 12 1 and

3 and provide quite long spans while still giving a level

soffit 13.

2.19 A banded plate is a solid slab, with concentrated bands

of reinforcement in place of beams. Whatever form it takes,

the load has to be got into the column head in the same

way as with the flat slab. Also, unless a trough-typo void-forming system is used, a grid ratio in the 1:2 range will

mean that the slab is spanning two ways and should be

reinforced. Many structures have been incorporated in

buildings with arbitrary one-way reinforcement and this is

not good practice, particularly when the slab is also carrying

the wind load in frame action.

13 Waffle pan shuttering, giving slab combining properties of

12a and b

3 Precasting

3.01 Precasting is a technique which has been in use for

very nearly as long as reinforced concrete itself. It can be

applied as a temporary site process or a permanent factory

process, to a standard or a one-off product.

3.02 The decision to precast is generally complex and

inevitably requires a degree of intuition and experience if

the choice is to be successful. The factors to be considered

are:

a, the ease or difficulty of formwork on site

b, the degree of repetitionc, handling or haulage to site or on site

d, plant, eg availability of cranes on the site

e, speed of erection required

Only once the project has been scheduled to start, and in a

decided form, can a decision be taken on whether or not to

14 Concrete Society award: North Thames (Jan Board,

Southend-on-Sea

precast. Provided the contract is to be negotiated with a

chosen contractor, the best solution can be based on the

contractor's existing resources or on what he can easily

obtain. The problems are greater if the contract is open for

competition. The designer cannot know how the successful

contractor will choose to do the work. An intelligent guess

has to be made in the design about the size of the members

and their distance from the probable crane position. This

may favour one tenderer while another would have submit-ted a lower price to a different arrangement. Unfortunately

designs are usually too far advanced at this stage to be re-

vised at a contractor's suggestion. On the other hand, where

time permits, contractors can often submit a precast

alternative for an in situ scheme at tender stage. However,

the more difficult the in situ formwork problem, the easier

is the decision to precast. Some projects are obvious targets.

The simplest decision is that of the floor or roof in a steel

frame or masonry structure. A quite different reason for

precasting in certain parts of the UK is the local attitude of

carpenters and steel fixers which may cause the contractor

to limit absolutely the number of employees on site.

Factory precasting

3.03 In theory the precast factory is ideal. It should offer

regular employment to an organised staff, minimum dis-

ruption by labour dispute and guaranteed delivery on time,

ease of construction and therefore maximum control of

quality and costs. In practice these are all variables and

when the building industry as a whole is at low ebb the

precast factory seems to suffer worst.

3.04 The standard1 product from the factory contributes

considerably to modern building mainly through the supply

of flooring units 15. These can range from a few centimetres

to more than 2 m in width, and up to 10 m in span. Most

units are shaped to give maximum efficiency for weight—important both in transportation and working.

15 Typical precast floor section*

3.05 The units illustrated 15 can be placed side by side,

requiring only a filling between them followed by a screed

ready to receive a floor finish. The keying between the unit

edges appears capable of spreading a concentrated load.

When the units are contained by reinforced concrete, by

main cross beams and edge beams, they can act as a struc-

tural diaphragm, imparting stability or carrying wind load

laterally to some stiff point in the structure.

3.06 Units can be erected by builder's hoisting wheel or

tower crane either individually or in batch and, depending

on size, manhandled or craned into final position. Since the

advent of prestressing in the late 1940s most manufacturers

have equipped themselves with prestressing beds up to

100 m long and techniques have been introduced in which a

machine moves along the bed, moulding the work and

leaving finished concrete which can be sawn into required

lengths.

3.07A variety of frames and members have been introduced

to capture the general building frame market, but successhas been limited. A standard system The Public Building

Frame, was promoted by tho Ministry of Works in the mid-

1960s but this found little favour outside the ministry itself.

The main reasons for the limited use of the factory precast

standard frame is that it imposes an immediate discip-

line in a material which for years has been advertised for its

design flexibility; also it demands a design crystallisation




much earlier in the programme than with the in situ struc-

ture. In development and industrial work decisions to

proceed are usually given too late relative to the required

completion date and, even when given, are liable to varia-

tion as the structure proceeds. Those circumstances favour

the more adaptable in situ construction.

3.08 Industrial single storey frames are a different propo-

sition. They provide a consistent cheap answer to the shed

type building as discussed later under skeletal structures(para 9.09). Other structural factory products include

sections of shell roofs, a complete range of standard beams

for bridges, box sections for culverts and manhole rings,

chimneys, staircases and cladding panels.

3.09 Apart from standard products, modern road and rail

haulage has enabled transportation of enormous members

over long distances. Although there is a range of standard

beams, motorway development has led to the production of

purpose-made whole beams, or components of hollow

girder bridges and arches.

Site precasting

3.10 For a multi-storey project with design for tableformwork there is no reason why in situ concrete should not

be the fastest mode of construction. Such schemes still have

edge beams, lintels, stairs and cladding panels and these

members have often been produced from a factory on site,

either by one central casting yard or a number of small

yards within crane reach of the building in question. Pre-

casting on site or nearby permits use of much larger

members since normal traffic requirements will no longer

apply. Hammersmith flyover 16 shows the scale of pre-

casting which can be achieved.

3.11 Shell construction becomes more feasible when the

complete shell can be cast in a site factory and lifted

bodily into position. This usually applies to northlightshells spanning between precast frames, for example the

Gallacher factory in Ireland. A similar process was used

on the Hull Technical College where one stressing bed on site

provided the entire precast roof system for the workshops.

At the main drainage outfall buildings, Beckton, part rings

of a barrel roof were precast, propped in position and

stressed together.

3.12 Precast units may act compositely as permanent

formwork for in situ concrete. In one method 17 columns

are constructed in situ.

3.13Precast beams are positioned on top and use either local

supports down the column face or odd supports along the

length of the beam. Precast floors are spanned from beam

to beam, and in situ concrete placed to the beam/column

connection and the whole area of the slab. Depending on the

form of the precast slab, virtually the whole value of anequivalent in situ construction can be achieved. Variations

on this theme include a variety of steel-to-steel connections

concreted into precast members. In one range of precast

products 18 a concrete soffit is provided containing the

main floor reinforcement such that the in situ concrete once

placed can work compositely to provide a finished structural

floor with many of the advantages of the fully in situ slab.

The soffit unit may be capable of spanning between supports

or may have to be propped temporarily to enable it to

carry the wot concrete and working construction load.

With the aid of hollow pots, concrete blocks, etc a wide

variety of the flooring units described previously (paras

2.01 to 2.07) can bo used in composite construction eitherwith or without temporary propping.

3.14 A recent addition to the precast standard range is the

double-T spanning up to about 15 m 19. Such systems can

be economic because of ease and speed of erection without

necessarily developing the full strength of an in situ struc-

ture of the same dimensions.

insitu concrete to columnsond to areas shaded solid

16 Hammersmith flyover: 16-span prestressed structure;

superstructure of precast units is prestressed to act as one

continuous member

supports to end

of precast beams

17 Composite structure: precast beams and floor units act as

permanent framework to in situ concrete

18 Composite structure: precast units with exposed

reinforcement act as permanent soffit to reinforced concrete

placed above

19 Composite structure: precast sections as permanent soffit

4 Jointing

4.01 The weak links in any structure are the site joints, so

joint control in both precast and in situ work is important.

The earliest attempts at precasting relied on a full dead

bearing with beams landing on corbels. In an effort to

improve appearance, beams were scarfed at their ends and

took the profile of the corbel. However, corbels are not easy

to cast and today beam-to-column connections tend to rely

on steel inserts cast in opposing faces and bolted or welded

together. If the connection is to bo neither too costly nor

too critical, the all precast frame must not rely on continuity

to the extent of the in situ construction, but can compensate

by using a higher strength concrete from the factory.

4.02 The column-to-column joint in multi-storey work has

always been a problem. One solution is to make the lower

half of the upper column hollow to receive the vertical bars

from the lower column. The void has to be successfully

grouted for the joint to transmit direct load and relies on




stability from some strong point such as a staircase enclo-

sure or crosawall. This led, before the Ronan Point collapse,

to some rather dubious constructions. A more positive

connection is the threaded bar, with right and left hand

threaded couplings capable of developing most of the

strength of the bars so joined. Up to four storeys it is

sometimes possible to cast the columns to full height and

then use beam-to-column steel connections to complete the

frame. Up to four storeys in narrow width buildings it maybe possible to precast a section of the elevation to full

height and one and a half bays width, thus including two

columns within the section with horizontal beam infills to

complete the elevational frame.

4.03 A final method of connecting columns is to wold the

steel at each column end to a steel plate, mate the plates

and weld them round the exposed perimeter. The final joint

stability in any arrangement of members can be achieved

by post-stressing, though this is only likely to be economic

in large span structures and is seldom used in normal

building work.

5 Prestressed concrete

5.01 Prestressing is the act of compressing a concrete

member by pretensioning its reinforcement. This enables it

to carry an amount of direct or bending tension without

the concrete going into tension or at least keeping the

tension within the allowable tensile stress. Three methods

are commonly used. Concrete may be cast around stressed

wires and the wires released from their anchorages after the

concrete has reached a strength of about 35 N/mm2. The

concrete grips the wires and is compressed as the wires tend

to return to their original length. Alternatively the wires or

tendons may be housed in a sheath and the sheath concreted

into the member. At a concrete stress of about 40 N/mm2

the tendon can be tensioned and retained by an end anchor-

age. Finally an external jack may be used, operating

between the member and a buttress, but this is most likely

to be employed to adjust a reaction in a redundant struc-

ture.

5.02 Stressed wires are commonly used in precasting

factories. A permanent bed is constructed on which a

variety of members can be cast in line, member size being

restricted by crane coverage. An early disadvantage was

that the wires could lie in only one position along the bed

and therefore the stresses at the end of the member were

similar to those at mid-span, thus losing some of the design

advantage in both bending and shear. Developmentsinclude deflecting the wires or coating them to cause

disbonding at the ends of a member.

5.03 The post-tensioning method is perhaps the most

flexible, assuming the tendon can be made to take a desired

shape and thus work to maximum efficiency. On the other

hand members have to be stressed individually and the

expensive anchorage units at both ends are permanently

lost within the concrete. Also the cable has to be grouted

and this can be a source of trouble if badly done. The main

advantage in the normal range of structures is that the

concrete is not cracked at the working stress level. This

protects the steel and gives a better deflection characteristic

because the full section of the member remains available

(normal re will be in tension below the neutral axis). Pre-

stressed concrete has come to be regarded as another

material but it is just the game concrete with another means

of reinforcement. With higher yield steels certain shapes of

member can be made more economically in ordinary

reinforced concrete, even with long spans. As ultimate

loading is reached there is not much difference between the

stressed member and the unstressed member containing the

same steel. Nearly all the notable successes in precast, or

in situ, prestressed concrete are in long span structures eg

bridges.

6 Sliding formwork

6.01 Originally developed for construction of silos ana

bunkers, it is now used in building the service, lift and stair-case cores for multi-storey buildings 20. Formwork about

1.3 m deep is erected at foundation level on the line of the

structure walls. Concrete is placed in the forms which aro

then raised slowly by a screw or hydraulic mechanism

arranged to climb vertical steel rods cast in the foundations

and extended upwards by couplers as the work proceeds. If

the weight of concrete in the forms is below a certain opti-

mum the concrete may be lifted by friction with the form; so

150 mm is about the thinnest section to use. Success depends

on uniform and consistent supply of concrete and construc-

tion is usually carried on non-stop for the full lift. Progress

is in the order of 150 mm to 300 mm per hour. Checks on

verticality are made as the height increases and the jackingis varied to compensate for any tendency to lean. Holes for

windows or doors, or pockets for beams, can be cast as

work proceeds.

20 Sliding formwork: services tower, Addenbrooke's Hospital,

Cambridge

7 Lifting and jacking

7.01 One answer to the formwork problem is to cut out

staging by doing the work close to the ground and then

lifting or jacking it into its final position. The most straight-

forward system is 'Lift slab' as applied to the column and

flat slab structure. Columns are first cast to part height, or,

if not too high, to the full height of the proposed structure.

Slabs are cast in their plan position at ground level, one

on top of the other, using a separating medium between the

layers for connection to the columns when the slabs have

been raised to the correct level. But because lifting is by

jacks at the column heads, care must be taken that tho

columns are stable throughout. Slabs can be constructed




as plate floors by any of the design methods describedearlier (paras 2.01 to 2.19). Jacking from below is usuallyapplied to whole units of beam and slab, or shell roofs. Thisstill requires formwork of the shape of the member, theobject being to achieve this on the ground rather than onstaging in the air, with resulting economies in formworksupport and concrete placing.

8 Tolerance

8.01 The greatest disservice to concrete is to expect it to beexactly as drawn. First there are the moulds-if the numberof members is less than 100 these are usually timber-whichwill not be perfect in the mould shop let alone after deterior-ation with use. Further inaccuracies may occur duringassembly. The entire formwork in an in situ scheme mustbe erected to plumb line and level, and so allowances mustbe made for inherent inaccuracies of measuring in space.

The formwork has to be held in position against constructionloads incurred in placing the concrete. The concrete thendeflects on striking and in time shrinkage and creep changethe deflection again.8.02 A sensible tolerance hits to be allowed. The tighterthat tolerance is, the higher the cost. An example of ill-considered detailing would bo to place a window unitbetween two concrete columns and horizontal memberswithout the minimal 15 mm clearance all round the theoret-ical concrete inner face. There may be greater accuracy inthe shape and size of precast members, but the actualassembly may demand even greater control to validate themethod of jointing.8.03Moulds used in precast work are likely to be accurate.However, subsequent reinforcing and handling, as well asthe results of over early demoulding and ill-consideredstacking and transportation, can play even greater dimen-

sional havoc than that found with in situ. Individual,small-section, pretensioned members are the worst, andtolerances should be carefully specified. Length, cross-section, straightness or bow, squareness, twist or flatnessshould be defined specially for the job in hand. In post-tensioned units particular reference should be made to thecamber and the variation of camber between adjacentmembers. There have been some unsatisfactory results inprojects where the only control of tolerance has been themanufacturer’s statement that the work would complywithCP 116 The structuraluse of precast concrete.

9 Building and structural types

The narrow depth building9.01 Speculative office building has led to many buildingsbeing planned on two office depths plus a central corridor—an overall structure width of from 10 in to 15 m. Framinghas usually been provided with one or two internal columnarelated to the corridor line and an elevation column moduleto satisfy the window arrangements. All the options of column, beam and slab structure described earlier are open.

The first solutions were conventional 21 but gave way tothe banded plate floor 22and, in cases of particular occupa-tion, to the clear span based on the deep ribbed floor orprecast, prestressed beams across the building carried by adeep beam set within the solid part of the wall elevation 23.

The single span building9.02 This follows logically from the narrow depth type. Inthe early ’50s secondary schools and technical collegesdemanded a two-to-four storey structure of one classroomand corridor width, usually about 10 m. This led to multi-

170

storey single bay portals, spanned with a variety of floortypes, usually ribbed for lightness 24. The portal frame issensitive to movement and may bo open to differentialsettlement and so needs careful consideration. One solutionin a difficult foundation situation involving deep piling wasto cantilever from a single central base. The externalcladding frame was hung from the roof -level beam25.



High-rise dwellings9.05 Replacement of war damage, slum clearance, and thedesire to provide open landscaped areas around housingsites helped to promote tower and slab dwelling blocks.Starting in the late 1940s on the column, beam, slab basis,these hives of flats and maisonettes rapidly progressed intocellular and crosswall structures rising to 20 or morestoreys. The introduction of the housing yardstick put apremium on minimum suporstructure cost, with littleregard to foundation cost. Many techniques were employedto rationalise the combined construction and finishingprocesses. In in situ work, shuttering developments- including the development of table top formwork-resultedin a sufficiently high standard of finish to both wall andceiling to allow decoration with minimum preparation.Getting the best from this type of construction requiresseveral disciplines. There should be no downstand beamswithin the building and preferably no edge beams. Thisallows maximum flexibility for tho table formwork. If edgebeams are required they should be upstands cast one floor

later, so that the tables can easily move out to the craneand upwards to the next floor. If an edge upstand or acombined cladding and structural member has to be con-

creted in at the same time as tho floor, the table legs can boarranged to hinge and clear the sill. Lintels should boavoided, allowing door units to go to the underside of the

9.07A large estate at Milton, Glasgow, shows the ultimatein rationalised tradition. Here a purpose-made steel tableformwork was designed for the complete floor of a tower.Walls were shuttered internally in large steel panel form-

work, and externally by precast, exposed aggregate,permanent formwork which could be placed by crane andstrutted from the inside of the building. The elevationswere designed with full height, slits enabling tho tableformwork. with side leaves hinged down, to be withdrawnon a special platform, and lifted by the crane. This slit waslater clad in glass and timber applied from inside thebuilding. No scaffolding was required.9.08 Industrialised building was a stop beyond rationalisedtradition in an attempt to provide an entirely precaststructure incorporating the maximum of finishes andservices. Although composed largely of reinforcod concrete,this type of construction is classed as masonry and will bedealt with in section 8 of the handbook.

Skeletal structures9.09 Where the formwork and its supports are large com-

pared with tho quantity of concrete contained, and wherethe structure is composed of single members of skeletal type,

in situ concrete is rarely economic. Hero precast work comesinto its own. Skeletal structures are primarily of thoindustrial or shod typo and cheap buildings can be providedin tho pitched roof form up to about 35 m in span29. Jointscan be made at convenient points and the structure analysedaccordingly, taking account of hinges or placing them at

171

The hull core building

9.03 Problems of vertical circulation, escape, services and

storage in office block developments with from 20 to 30

storeys led to the grouping of these facilities in one or two

areas on plan which could be protected by reinforced con-

crete walls through the full height of the building and able

to cantilever from the base for stability 26.

9.04 The structure outside the cores could be designed with

primary concern for vortical loading, the columns takingonly local bending from wind-load could be of minimum

section. The conventional vortical wall formwork, con-

structed floor by floor, first gave way to larger panels. The

most recent development is construction of the inner core

by sliding formwork, enabling the whole of the inner cores

to bo erected as towers at the rate of 150 to 300 mm per hour.

Deep cantilever frames coincide with each service floor.

Suspended hangers provide tensional support to the peri-

meter of the building 27. It is early to predict whether

these latest developments are economic. Fashion in struc-

ture as elsewhere is not always well-founded.

26 Hull core building. Stability is from vertical reinforced

concrete circulation and service cores cantilevering upwards

from base

1 2 3 4

27 Hull core building. Cantilevered horizontal frames at each

service floor. Suspended hangers tension building perimeter


slab above. Selected walls may be linked by heavily re-

inforcod lintels to provide stability under wind load.

9.06 A particular example of this is the Borough of Wands-

worth's Surrey Lane Project 28 of four 21-storey towers. For

economy, the entire structure including elevations is in

lightweight concrete. Precast edge members were hoisted

into position on the table formwork and made continuous

with the slab. Walls 175 mm thick, even in lightweight

aggregate concrete, give adequate sound reduction and fireprotection between tenancies and also the fire protection

required around lifts and stairs.

28 Surrey Lane project, Wandsworth. Precast edge beams

occur on two opposite elevations, enabling in situ edge beam

shuttering of other two elevations to span between




the points of contraflexure. Light roof spans up to 16 mcan be achieved with trusses of traditional steel shape,while longer spans and heavier loading require heavier,probably prestressed, trusses 30. Purlins for these skeletalroofs can be made in a variety of shapes 31. The rectangleis not the best structural section but,LS and TS presentproblems both during manufacture and in the deflection of sloping roofs. Plugs can be cast-in for cladding connections

or, in the case of lightweight concrete, masonry nails willsuffice.9.10 Space frames in concrete are essentially single layergrids and can be in situ, part precast or wholly precast andthen post-tensioned together 32. Although some structureshave been built in this form, developments in single, doubleand triple structural-steel layer grids make concreteuneconomical. The fire resistance value must be of greatimportance to justify concrete in this form 30 as structuralsteelwork, with its low weight/strength ratio, is the basiceconomic solution.

Surface structures9.11 The primary purpose of the shell form of construction

is to use the cladding surface as a structural member 33.Suitable curving or corrugation of the surface can produceefficient structural form, so concrete with its mouldabilityimmediately suggests itself. Some exciting forms of shellhave been constructed, particularly in South America andSpain. While in Britain the story follows the familiarpattern, fashion is often the only reason for the adoptionof an innovation and so misuse quickly leads to disrepute.9.12 In the late 1940s ‘barrel’ and northlight cylindricalshells were represented as an economic solution to allroofing problems and in early work exaggerated claims of low cost led to inappropriate applications34. Certainly, oneor two projects with adequate repetition resulted in satisfac-

tory costa, but single shells have high formwork, setting-up,and distribution coats (remembering the small quantities of concrete involved). Certainly, the minimum support roof could be visually exciting, but in the industrial field ittended to restrict the flexibility of buildings.9.13 A ‘barrel’ roof spanning an area of 50 m by 25 m wouldhave a dead load of 320 kg/m2, about four times more thanthat of an equivalent steel-framed metal-clad roof whereboth are designed to carry 70 kg/m2 superimposed load.Foundations must obviously be urgently considered in theoverall costing. Theshapingof the shell from which strengthis derived may well increase the volume of the building andtherefore the heating load.




9.14 With rapidly rising labour rates, formwork costs for

individual shells are likely to make them uncompetitive, but

cost is not always the final consideration. If all the require-

ments of the project indicate a shell, namely the need for an

enclosed envelope or the use of large unloaded spans, then

a wide variety of shapes are available.

35 Surface structure: square dome, warped and barrel

9.15 Meaningful calculation may only be possible with

certain shapes but testing of models in micro-climate with

fine wire reinforcements can demonstrate to an engineer the

manner in which the shell behaves under load. Considering

first the simpler form of cylindrical shells, the 'barrel' and

northlight were investigated in depth in the early 1950s 34.

9.16 Whether an edge beam is used or not, the rise to span

ratio should be about 1:10 35. To obtain this depth an edge

beam is necessary with a narrow barrel but as the width is

increased the appropriate depth may be obtained in therise of the shell itself. Compromise between these require-

ments and the cost of the columns suggests a span to width

ratio of 2:1. Typical proportions in this range are shown 36.

End diaphragms must be provided and can take a variety of

forms 37.

9.17 As in structural steelwork, northlight shells provide

certain advantages but at extra cost. The quantities of

concrete and steel are greater than in the 'barrel' of similar

span; also unit construction costs are higher. It is usually

difficult to obtain as large an effective depth (see ANALYSIS 2

AJ 24.5.72 p1171), and the beam below the glazing, which

forms the gutter, tends to carry a high proportion of the

load. To increase the depth of construction the width mustbo not less than half the span, relating as shown 38.

s

30

50

W

(mm)

1 5

2 0

25

R

(m)

7

15

18

t

(mm)

60

70

80

approximate weight /m

(including mm live bad (kg)

270

3 0 0

350

3 9 0

36 Barrel structure, properties

37 End diaphragms commonly used with barrel structures

38 Northlight shells: typical proportions

9.18 Square areas can be covered by a square 'barrel',

though this is not likely to be economic. The alternatives

are two or three 'barrels' of less rise carried on a common

end beam, or a square dome—ie a barrel curved in two

directions 35. Such a shell can theoretically span up to 100 m

with little more than 100 mm thickness. The more recent

fashionable shell is the hyperbolic paraboloid—effectively a

warped parallelogram or a surface of translation 39. In the

latter case the surface is obtained by moving a vertical

parabola having upward curvature over another parabola

with downward curvature, the parabola of translation

30

30

10

11

100

(m)




lying in a plane perpendicular to the first but movingparallel to it. This is shown graphically where the saddleshaped surface is formed by moving parabola ABC overparabola BOF.9.19A variety of roof forms may be developed either by useof the entire warped surface or by combining parts of it invarious ways. The supports shown, if too flexible, would betied across to avoid the spread of the arc 39. The complete

warped surface has been used with striking effect) in chur-ches, restaurants and roofs of garage. Structures formedby combining four stiff quadrants at angles to one anotherare suitable for covering the large rectangular areas commonto industrial plans. An alternative grouping produces theumbrella structure. This can be horizontal or tilted -if tilted the units may be combined to produce a northlight,roof effect 40. The span/rise ratio is usually taken as notmore than 9:l. Typical sizing for an umbrella is shown.9.20 Though tho double-curved surface appears a difficultformwork problem, it in fact requires only straight linegenerator wood joists on which the warped face can beachieved by nailing flexible plywood sheathing. Stresses ina ‘properly proportioned roof are low and 75 mm thickness

suffices for the normal span. The University of Mexico hasone roof only 16 mm thick but in theUK 60 mm is about theright answer for steel reinforcement mesh plus cover.9.21 The folded plate structure differs from the shell in that,it is built up from slabs, and not necessarily thin as in themembrane. These carry bending as well as direct and shearforces. Prismatic structures consist of rectangular plateswhich are immovably restrained in relation to one anotherby means of transverse stiffening diaphragms or rigidframes. The roofs illustrated fall into this category 41.Pyramidal structures occur as silos, cooling towers andpeaked roofs 42, while the prismoidal structure falls be-

tween the two, being a sawn off version of the pyramid43.9.22However, as every slab-to-slab junction can be regardedas a beam support almost any combination of folded plateswill provide a stable structure. The scale of the platesshould be such that the so-called ‘beams‘ have span todepth ratios of less than 2:1 and are designed as ‘deepbeams’—as distinct from normal beam theory. Formworkis conventional and can be made up from panels as large ascan be handled on site.



1 Introduction

1.01 Design variables in reinforced concrete are many ; aridalthough attempts have been made to produce designcharts for reinforced concrete sections similar to those forstructural stoolwork, design is still apparently done eachtime from first principles. Perhaps it is this which causesarchitects to regard all concrete design as tho province of the engineer—but there is no reason why an architect,should not be as prepared to do simple design in concreteas he is, stay, in timber.1.02 OnI projects where no engineer is employed, simplesolid floor slabs, stairs, simple beams, lintels and small

retaining walls could well bo dealt with in an architect’soffice. Apart from his ability to do this work, an architectoften has a responsibility in site supervision and should boaware of good practice in steel detailing and fixing, as wellas legislation governing tho general use of concrete.1.03 In Technical studySAFETY 1, the modern approach todesign was discussed; for concreto, the now unified code of practiceCP 110 will apply in 1973 but this does not concernarchitects’ design—-indeed many practical engineers con-

sider it is not even a code of good practice. However theexisting Code (CP 114l) will remain in use for some yearsand, provided no condition of the unified code is moreonerous, there is no reason whyCP 114 should not be usedindefinitely for simple designs.

2 Stress and strain

2.01 Section 2 of the handbook (Structural analysis)showed how structures are analysed and how the direct

forces, bonding movements arid shear forces induced in thestructural members by the applied loading are determined.In designing a reinforced concrete member it is necessaryto place the reinforcement in tho section so that it anti-cipates the tensile stresses which the concreto is not ableto accept. The following paragraphs (2.02 to 2.21) explainthe stress conditions in tho concrete beam or column anddefine the allowable stresses permitted it, CP 114.

Beam or slab in bending2.02 Early design was based on the assumption that con-

crete was elastic in the working stress range and the ratioof the modulus of elasticity of the steel to that of concrete

was 15. In other words the approach was that an area of steel contributed 15 times that of the same area of concrete. This had the effect of penalising higher strength steels aridpreventing their economic use.2.03However tests proved that as failure was approached,the compressive stresses in a member, instead of being amaximum at the edge farthest, from the neutral axis,adjusted themselves to a nearly even value from neutralaxis to extreme fibre, giving a total compression greaterthan that assumed in the elastic theory. This stress blockresulted from the concrete assuming a 'plastic' rather thanan 'elastic' state.2.04 The 'plastic' design permittedin CP 114 accepts theconcept, of a rectangular stress block, as in 1, provided that,

its depth is not greater than half the depth from extremecompression fibre to the centre of the tensile reinforcement.2.05Forces at working load are shown in 1 and this assumessteel reinforcement in compression as well as in tension.(Compression steel is ignored for the time being.) The tensile


Technical study

Concrete 3Section 5 Structural material: Reinforced concrete

Simple design inreinforced concrete

ALLAN HODGKINSON'S thesis in this final study on structural

reinforced concrete is that an architect need not be afraid of

designing simple concrete members himself, and that

certainly he should be aware of what such design involves.

Assuming an understanding of the analysis (section 2 of

the handbook), this study explains the stress conditions in

loaded rc sections and tabulates those permitted; it touches

on cover, bar spacing and explains reinforcement detailing.

Finally examples of typical structural situations are

worked through in detail to illustrate the earlier sections of

the study. Cover and bar spacing, with all code

requirements, are detailed in Information sheet CONCRETE I

which follows this study




force (T) is the product of the permissible tensile steel stress(Pst) and the area of tensile steel (A st), i.e:

and the compressive force (c) is two thirds of the permissibleconcrete stress (Pcb) multiplied by the stressed area (bbreadth of section and dn depth of the compressive stressblock); ie:

T = Pst Ast

2.06 If this maximum is more than that required to resisttho external moment, then a stress block of less depth than

if the concrete is still stressed to the allowable

value the lever arm is thorefore greater and less steel willbe required. A graph 2 can bo plotted to express this condi-tion.

2 Lever arm curve for load factor design. Graph of lever

factor exceeds 0.125, compression reinforcement is required

2.07 In order to calculate the tension steel required it istherefore necessary to derive the appropriate value of

(b and d1) and tho permissible concrete stress (Pcb). Tho leverarm factor (y) can then bo read off tho graph. The tension

would be deeper than the permitted half-section. In thiscase it may be possible to provide tho extra resistance byusing compression steel. The lover arm to calculate thoamount of steel is the distance between the compressionsteel and the tension steel (d1 - d2) and

That is, tho required compressive resistance divided by theproduct of lever arm and permissible steel compressivestress (Psc). This amount must not exceed 4 per cent of thearea of the member section; if it does, then a deeper sectionin required.

Rectangular sections under direct compression2.09 Tho plastic properties of concrete under stress have,for a considerable time, been used in calculating axial load

resistance in columns. The safe load can bo easily assessed as

where A, is area of concrete, Pcc the allowable direct stressin compression in concrete; A,, is area of steel and Psc

allowable compression stress in the steel.2.10 Although the term ‘crushing’ is used in concrete testingthe concrete cube fails on diagonal planes and if thistendency can be resisted by some binding force the concrete

can carry enormous compressive stress. Thus while rectan-gular steel links are usually employed to prevent a columnbar from buckling, the use of helical binding would allow ahigher safe load to be calculated.

Ac Pcc + Ast Psc

Rectangular section under direct and bending stress2.11 The plastic design (load factor method) for bending anddirect stress is an extension of the process defined earlierfor beams. When the magnitude of tho bending momentrelative to the direct load is such that primary failure of themember would be by the yielding of tensile steel, theassumed stress distribution for resisting the working loadsis as shown in 3.2.12 The formulae presented in the code are far too com-

plicated to use each time a section has to be designed; butmaking the assumption that equal reinforcing steel will beused in each of the two bending faces of the member, graphscan be produced which show a series of curves plotted torelate direct load, moment, section width and depth, andcover to main reinforcement and allowable steel stress (allas related to any concrete quality). A typical graph isshown in 4 for the particular (d1/d) ratio shown inset. Withso many variations, it is necessary to have a family of graphs, so that each ono expresses a different ratio of effective depth of tensile steel to total section depth; assteel has been chosen to be symmetrical this means that thecurves are constructed in direct relationship to tho width

and depth of section.

graph can be plotted. This lies between curves 4 and 5, say4.4, When this is multiplied by a constant for the graph, itgives r, the total percentage of reinforcement in the column,

2.13 Other curves can be drawn for differing configurationsof reinforcement withd1/d = 0.7, 0.85, 0.9 and 0.95 (see 5). Curves can also bedrawn for circular columns and for columns subjected tobonding on two axes at right angles.

3 Load factor method: stress distribution in rectangular

section under direct load and bending at working load(figures are all defined in text) M = Pe




Shear stress2.14 A considerable number of research workers havecontributed to the subject of failure in shear and theInstitution of Structural Engineers issued a report in 1970on tho assessment of recent work2. Tho still unpublished

unified code approaches the subject in a similar way andprovides some economy in design compared with CP 114.However, as was said earlier, CP 114 remains adequate forsimple designs.2.15 Failure in concreto with no vertical steel will occur asshown in 7 when the principal stress developed across aplane at approximately 45° exceeds the tensile strength of tho concrete. Provided the average shear stress q does notexceed the allowable shear stress the section is assumed tobe adequate and q is givenby:q =Q/b 1a,where Q is applied shear, b width of section and 1a thelover arm.2.16 If the allowable stress is exceeded then links or bent up

bars must be added to the member to provide a latticebeam action with vertical tension members and inclinedcompression bands6. If bent up bars are used they lie moreor less in the direction of the principal tensile stress. Linksshould be provided at a spacing of

where Aw is tho cross sectional area of the link and Pst thepermissible stress in tension for shear. For given permissiblesteel stresses tables can be found in any concrete designer'shandbook with bar diameter plotted against spacings toshow the resistance value of the two logs of the link where

Band stress2.17 Only resistance of concrete to horizontal shear stressesand the adhesion of concrete to stool reinforcement allowthe compression and tension zones to co-operate in produc-

Permissible stresses2.21 The actual stresses allowed for concrete and steel instructural reinforced concrete are given in CP 114 andexemplified in tables I and II (considering for this purposeonly tho two grades of concrete most likely to be used

6 A normal beam with bent up bars or inclined compression

bands to counteract shear stress acting as in 7

7 Shear failure of concrete

ing the resistance moment of a beam.

The vital bond is difficult to assess numerically as it is a

combination of adhesion, the gripping effect of shrinkage,

the shape of the bar section and the form of anchorage.

2.18 The grip of the concrete on the steel preventing slippage

equals the difference in total steel force at any two adjacent

sections. This force (dst) equals the product of shear stress

(q), breadth of beam (b) and length (dl), ie dst = qbdl.

2.19 Assuming local bond stress of uniform intensity, the

total steel force also equals the product of bond stress,

perimeter of all the bars (o), and dl.

q b dl q b.

o d1=

oTherefore local bond stress =

But, as q was defined in para 2.15 as q = Q/la.b, finally

local bond stress = Q/la.o. A permissible stress is givenin CP 114 for each quality of concrete.

2.20 In addition, for anchorage or lapping, the bar must

extend from its particular point of stress a distance in

tension of:

tensile stressbar diameter × ——————:—:—————————————————

4 × permissible average bond stress;

and in compression of:

compression stressbar diameter × —————————————————————————

5 × permissible average bond stress.

In practice particular combinations of concrete and type

of steel bar will require a certain number of diameters

of length for anchorage when using the full permissible

stress in the stool. It is then easy enough with this figure

in mind to design the correct bond length or reduce it

proportionately for lessor stresses. A reduction in the length

is made for L-hooks or U-hooks at the end of bars as in 9.

intensify o|

shearSection Elevation distribution

8 Bond stress diagram* shear on this horizontal

plane must balance the

bond in the bars

4 Typical graph for determining steel reinforcement

required in the rectangular column shown inset (d1 /d = 0. 8)

5 The expression

within the section; three examples

Qthis is designated as — .

(

pst Awlas =

Q

d describes the relationship of bars

d 1)—

la

( )




1:2:4 concrete at a 28-day cube strength equal to 21 MN/m2

and 1:1½:3 equal to 25.5 MN/m2).

9 Reduction in length for bond-permissiblewithL- or U-

hooks at end of bars (D =bar diameter)

3 Cover and bar spacing

3.01 Earlier in this handbook (Technical study SAFETY 3)the requirements to protect reinforcement in the event, of fire attack were defined. Reinforcement must have adequatecover both to protect it from weather and to give it adequatebond, and it must be distributed in such a way that crackingdue to shrinkage and temperature movement of the con-

creto is controlled. Good detailing of the reinforcement is just as essential as calculating correctly the right amountof reinforcement.3.02 The maximum size of the aggregate from which theconcrete is made demands some control over the cover andspacing of bars. Information sheet CONCRETE 1, whichfollows this study, illustrates the code requirements andrelates to 20 mm maximum size aggregate.

4 Detailing and scheduling the bars

4.01 The main principles of good detailing are as follows:a, to depict in tho drawing and schedules the number andshapes of bars so that the designer’s intention is properly

interpreted and the steel-fixer can fix the steal in the samemannerb, to put in just sufficient reinforcement to guard againstshrinkage and movement cracking, and to allow the bars tobe caged together when only the steel required to support

the structure is indicated in the calculationsc, to detail the bars in member sections so, that there are nosudden changes from heavy to light reinforcement andno areas of concreto completely unreinforced.4.02 Detailing should be carried out in accordance withBS 4466. In general, this involves the use of standardshapes (each of which is given a referonce number known

as shape code) and a list of dimensions A, B, C, D, E or Rwhich are written into the schedule in place of the barsketch.4.03 Two tables of standard bar shapes are given inBS 4466 Table 6-preferredshapes and Table 7-othershapes;those shown in table 6 should be used wherever possible.If a bar is required which is not covered by a standardshape, ‘99’ is entered in the shape code column and a sketchof the bar is made in place of the standard dimension A,

4.04 The dimensions of standard shapes to be entered in thebar schedule are restricted to those shown in the fourthcolumn of (BS 4466) tables 6 and 7. This means that theclosing dimension is not given. Note that non-standard radii

are not always shown as such but may be indicated as anoverall dimension (eg shape 36).

B, C etc 10.

Tolerances4.05 Cutting and bending tolerances, shown in table I of BS 4466, should be noted carefully, and when detailing thebars on drawings, account should be taken of them. Inparticular note that the tolerance on tho length of straightbars is ±25 mm, and that on normal links (up to 1000 mm,maximum side dimension) is ±5 min.4.06In special cases, where the tolerances in tableI (BS4466)could lead to an unacceptable reduction in cover and wherethis is critical (eg precast concrete cladding panels), strictertolerances can be adopted for particular bars; this must bostated on the bar schedule. As this will undoubtedly

increase the cost of the reinforcement, it is to be used onlywhere absolutely necessary.

Bar marks4.07 Bar marks are retained as plain serial numbers 1, 2,3 etc, starting at 1 on each drawing. Type and size areentered in the adjacent column on the schedule, the sizebeing the number of mms in nominal size commonly usedin the round and square ranges of areas (see clause 2.2 of BS 4466), and the types most commonly used beingR (mildsteel) and Y (high yield steel), both in the round range.Square twisted bars or bars not covered by R or Y aredenoted x.

5 Examples

Floor slab5.01 The first of three examples covers the design of a four-span continuous floor slab carrying a 50 mm screed, light

Mix

proportions

1:2:4

1:1 :3

Permissible stresses (in MN/m2 or N mm )

Compressivedirect

5.36.5

bending

7

8. 5

shear

0. 7

0. 8

Bond

average

0.830.93

local

1.251.40

Type of

stress

Tensile stress

other than in

shear

reinforcement

Tensile in

shearreinforcement

Compressive

stress

Permissible stresses (MN/m2)

Mild steel bars

diameter diameter

<40 mm >40 mm

140

140

125

125

125

110

High yield steel

having a guaranteed

yield or proof stress

(f y)

0.55 fy but not more

than 230 for bars not

exceeding 20 mm dia

0.55 fy but not more

than 210 for bars not

exceeding 20 mm dia

0.59 fy but not morethan 175

0 .55 fy but not morethan 175

10 Bar schedule example

where fy = guaranteed yield or proof stress or specified characteristic strength.

Table II Permissible steel stresses ( from CP 114)

Table I Permissible concrete stresses (from CP 114)

2




weight partitions, plaster to the soffit and a superimposedload of 2.5 KN/m2 11.

The span dimensions used in design is the lesser of a, centreto centre of supports or b, clear dimension between supportsplus effective depth of slab; the depth of slab must not be

Effective span for end bay =5000 +175 =5175;effective span for inner bay =5200 +175 =5375.Loading: 175 mm slab 4.2 KN/m2

finishes 1.5partitions 1.0superimposed 2.5

giving a total of 5.02 The code (CP 114) gives an approximate analysis forcontinuous slabs of three spans or more when the spans donot differ by more than 15 per cent in length. This analysisis quite adequate with spread loading and nearly equalspans, and there is no need to do any more complicatedanalysis. The moments (assuming Wd = total dead loaddistributed; Ws = total superimposed load distributed; and1 =effective span) arc given in table III.

Table 111 Moment formulae

9.2 KN/m2.

In this particular case they give the figures in table IV(where all moments are in KNm per m width).

At each of the four points at which the moment has beencalculated, it is necessary to road from the graph of lever arm

correct lever arm for each case. This, taking permissiblesteel stress (Pst) to be 230 MN/m2, the area of steel (Ast) iscalculated, as in table v

TableV Steel required

5.04 Distribution steel must be placed across main rein-forcement partly to distribute the load evenly, but also toavoid shrinkage cracking. With high yield steel, the distri-bution steel area should bo at least 0.12 per cent of thecross sectional area of slab,

Therefore use 10 mm at 300 mm centres for both top andbottom steel.5.05 Tho shear can be tabulated as in table VI. Resistanceto shear =0.7 X 0.15 X 0.87 X 1000 =91.

Table VI Shear

5.06 This calculation was carried out to show the procedure;but with solid slabs, unless subjected to superimposedloads much higher than the dead load, shear stress isunlikely to exceed permissible stress.

5.07A section through the floor would be dotailed as in13.At A, alternate bars from the bottom are taken round andinto the top of the slab to provide a nominal resistingmoment to counter the fixing moment induced by clampingaction of the wall above (though this end was treated as

simply supported in the design analysis). Chairs, as14, arerequired at internal supports to keep the top steel in itscorrect position and should bo detailed on bar schedule.5.08 Letters on 13 are bar marks, and these define the barson the bar schedule. Dimensions are required to indicatethe amount by which the top steel (e) over the supports is

Moment

KN/mm run

21.52

14.06

25.22

M

pcb(bd 12)= 0.00637M

0.137

0.09

0.161

Lever

armfactor

(γ )

0.88

0.93

0.86

Ast = moment

Pst (γ ) d1

in mm2/m width

21.52 × 1000

230 × 0.88 × 0.15

= 712

14.06 × 1000

230 × 0.93 × 0.15

= 437

25.22 × 1000

230 × 0.86 × 0.15= 845

Use

steel

12 mm

bars at

160 mm

12 mmbars at

260 mm

12 mmbars at

135 mm

Shear

Static load

Change in

load due to

moment

Total

Support A

2.5 × 9.2 = +23.0

25.22

5.075= -4.9

+ 18.1

Support B

span

AB

+ 23.0

+ 4.9

+ 27.9

span

BC

+ 23.9

+ 0.2

+ 24.1

Support C

2.6 × 9.2=+ 23.9

25.22 + 24.1-

5.375= -0 .2

+ 23.7

Mid AB

Moment due to

dead load

Moment due to

superimposed load

Total

+ 14.84

+ 6.68

+ 21.52

Support B

-17.80

- 7.42

-25.22

Mid BC

+ 8.05

+ 6.01

+ 14.06

Support C

-16.10

- 8.00

-24.10

Moment due to

dead load

Moment due to

superimposed load

Middle of

end span

+

+

Wdl

12

Wsl

10

Penultimate

support

-

-

Wdl

10

Wsl

9

Middle ofinternal

span

+

+

Wdl

24

Wsl

12

Internal

support

-

-

Wdl

12

Wsl

9

Table IV Actual moments for this example

5.03Use 1:2:4concrete, 21 MN/m

2

at 28 days, pcb = 7MN/m

2

.MM

Pcb(bd12) 7 × 103 × 1 × 0.15

2 = 0.00637M. (b and d1,

as in 12).

less thanspan

31.5 if high yield steel is used as reinforcement.

In this case,

span 5200 + say 200= 172 mm; use slab depth of 175 mm.

31.5 31.5 ie0.12 × 1000 × 175

100= 210 mm

2per metre width.

factor (γ ) againstM

Pcb(bd 12)

, the correct γ factor, and thus the

-

=

( )



Technical study Concrete 3 para 6.08 to References 180

staggered in alternate bars. The bars must develop theiranchorage bond length from the point of maximum stress,but as the bending moment falls sharply 15 the first barcan be curtailed quickly, and the alternate bar then carriedon to just beyond the quarter span point.

Continuous footing5.09 The second example is design of a continuous footing.Assume that the wall carries 260 KN/m2 run and the soil hasa permissible bearing pressure of 216 KN/m2; use high yieldsteel and 21.0 MN/m2 concrete 16.

Width of footing required say 1200 mm. The wall loadspreads down to the level of the reinforcement as shown in17, so the critical position for shear failure is along line A.(Depth to reinforcement = 240 mm.)Load to left of A = 0.235 x 216 = 51 KN per m width. This is obviously quite safe with a dab depth of 300 mm.

5.10 Considering the ground pressure as a load on a canti-lever springing from the centre line of the wall, the bendingmoment at the centre line of the wall

5.11 Use 12 mm bars at 150 mm centres and distributionsteel of 12 mm bars spaced evenly in the width of the base.In foundations bond length should always be checked foranchorage from maximum stress position to end of bar andif large bars are used the local bond should also be checked.In this particular case the bar length measured from thecentre line of the wall is about 50 times the diameter of thebar and therefore acceptable. The design would be as 18.Note that a 60 mm blinding concrete has been shown fortwo purposes. It both protects the trench bottom fromweather after excavation and also allows a clean surface onwhich to place the steel.

Lintel5.12 The third example is the design of a lintel. Assume thatthe lintel, carrying brickwork and roof trusses, spans 2.3 mand supports a spread load of 35 000 N/m including its ownweight 19. Use high yield steel end 21.0 MN/m2 concrete.

Effective span = 2300 + 230 = 2530 mm = 2.53 m

Shear force at support = 1-15 x 35 000 = 40 250 NShear resistance = 0.7 x 10002 x 0.267 x 0.85 x 0.230= 36 540 NShear reinforcement is therefore required. The spacing for 2 legs 10 mm links is

Say 100 mm links at 150 mm centres for 500 mm then theconcrete will be able to carry the shear; so use the same linksbut spaced out to 300 mm.

5.13 In practice, by the use of designer handbook tables,shear links could have been found by calculating the value

This value would be read off from the table as requiring the10 mm at 150 mm centres as already calculated. The beamwould be detailed as in 21.

1 BS CP 114: Part 2: 1969 The structural use of reinforcedconcrete in buildings. Part 2 metric units. £3.002 INSTITUTION OF STRUCTURAL ENGINEERS The shearstrength of reinforced concrete beam. 1969. o/p3 The empirical method to CP 114 is no longer applicable.4 Concrete admixtures, use and applicating. M. R. DIXON.

The Construction Press

References



181 Information sheet Concrete1

Information sheetConcrete 1

Section 5Structural material : Reinforced concrete

This sheet, to be read with Technical study CONCRETE 3,gives the code requirements from BS CP 114, and relates

to 20 mmmaximumsize aggregate. A and B show typical

requirements, C to G show special situations which an

architect may encounter

Cover, bar spacing and laps

A Typical cover and spacingrequirements for beams and slabs

B Typical cover and spacingrequirements for columns

Notes•a is general case, with minimum main bar size of 12 mm; b is special case,with columns of 200 mm or less width and where bars do not exceed 12 mm.•The total main reinforcement area should generally be at least 0.8 per centof concrete area; preferably it should not exceed 4 per cent of concrete area, and

it definitely must not exceed 8 per cent. No lap between upper and lower barsto exceed 8 per cent of concrete area.• Links are to be spaced at the least of the following dimensions:Least lateral dimension of the column, 12 x diameter of the bar linked, or300mm.• Link diameter to be not less than 5 mm or less than ¼ of the main columnbar dimension. Every main bar to be linked in two directions unless other-wisedirected.

C Concrete members exposed tomoisture : open platform

Note:With asphalt tanking, waterproofing membrane orsurfacing coversmaybe reduced to normal as in AorB



Information sheet Concrete 1 182

D Concrete members exposed tomoisture: exposed column and edgebeam

E Concrete members exposed tomoisture: exposed walls and stairs

F Cladding panels

Notes•Aggregate between 10 mm and 20 mm if used to be treated as large.•The use of heavily galvanised or stainless steel reinforcement can reduce thecover to as little as 20 mm•If stones are not well bonded A must be 40 mm minimum, otherwise Bmust be 40 mm minimum.

G Special considerations of cover

Notes•Lightweight aggregate concrete when exposed to moisture requires 10 mmmore cover than any cover dimension given in this sheet•Add 10 mm extra cover to exposed aggregate faces, or where retarders(Redalon etc). bush hammering or grit blasting has been used.•With water-retaining structures, walls and slab above and below ground,40 mm cover is required for all reinforcements in faces against water or earth.Also 26 mm minimum cover is required for any bar in a slab soffit over the tankwater.



AJ Handbook of

Building Structureedited by Allan Hodgkinson

In its first edition, this Handbook became a standardreference for both students and practitioners. Re-cent changes to British Standards, Codes of Practice

and Building Regulations have generated demandfor a new, updated edition; and unlike the reprints of

dated version of the original 1974 Handbook.

The principle changes are in the sections on Mas-onry (totally rewritten to take account of the 1976Building Regulations) and on Timber (substantiallyrevised to take account of new timber gradings). Inaddition to many minor improvements, the oppor-tunity has also been taken to bring up to date all thereferences quoted.

1976 and 1977, this is a radically revised and up-

For the rest, this remains the widely acclaimedstructural design handbook first published in 1974.

Information is specific enough to be of practicalvalue, yet presented in a way intelligible to userswithout engineering backgrounds.

Some press comment on previous editions:

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Of related interest

AJ Handbook of Building Enclosureedited by A J Elder and Maritz Vandenberg

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ment" IHVE J ournal

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interest to anyone concerned with the built environ-

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Paper edition ISBN 0 85139 282 2

Guide to the Building Regulations1976 (Seventh Edition)A J Elder

This new 1982 edition of the Guide to the 1976 Build-

tary of State's long-awaited Command Paper, in-corporates two new appendixes: on the ProposedSecond Amendment, and on The Future of BuildingControl in England and Wales.

Some press comment on previous editions:"An invaluable source of guidance through the ver-bal jungle of the Regulations"

Building Technology and Management

"The book provides a comprehensive reference onmatters of everyday practice for all members of thebuilding team and students and should act as acompanion to the 1972 Regulations themselves"

Building Trades J ournal

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Construction News

ISBN 0 85139 850 2

ing Regulations, coming on the heels of the Secre-

New Metric Handbookedited by Patricia Tutt and David Adler

With sales approaching 100 000 over the past 10years, the original Metric Handbook is an estab-lished drawing board companion. But now that themetrication programme in the UK is virtually com-plete, the emphasis on conversion to metric whichformed the basis of the old Metric Handbook is no

longer appropriate. This radically revised and great-ly expanded New Metric Handbook retains many of the features of the old, but concentrates much morestrongly on planning and design data for all com

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